Generally, a spindle unit used with a machine tool includes a clamping means for clamping a tool or a workpiece pallet in a hollow portion of a spindle mounted rotatably in a housing. A conventional spindle unit is described, for example, in FIG. 1 of Japanese Patent Publication No. 2-49843 or FIG. 7 of U.S. Pat. No. 5,222,285. A clamping means for clamping a tool or a workpiece pallet is configured of a draw bar inserted into a hollow portion of a spindle and a disc spring for applying an urging force to the draw bar. A collet is provided at the forward end of the draw bar to hold a pull stud mounted on the tool or the workpiece pallet. A plurality of disc springs mounted concentrically on the draw bar cooperate to apply the urging force to the draw bar so that the tool or the workpiece pallet can be clamped at the forward end of the spindle. When releasing the tool or the workpiece pallet, a cylinder-and-piston mechanism constituting an unclamping means and provided rearward of the spindle pushes the draw bar forward to thereby release the tool or the workpiece pallet from a mounting means provided at the forward end of the spindle.
The clamping means provided in the hollow portion of the rotary spindle of the spindle unit, i.e. a mechanism including the draw bar and the disc springs is supplied with a lubricant, in order to lengthen the life of the unit by reducing the wear on, and breakage of, each disc spring. In the prior art, the method of lubricating the clamping means include a grease filling method in which grease is filled, a lubricating oil circulation method in which lubricating oil is circulated and a lubricating oil filling method in which lubricating oil is filled in an oil pool. However, in the grease filling method, the grease is splashed off by the high-speed rotation of the spindle or melted by the heat generated by the spindle and, therefore, the spindle is not lubricated sufficiently. As a result, the disc spring is worn or broken, and the unit life is shortened. The lubricating oil circulation method has a disadvantage that it is very difficult to supply the lubricating oil from an external source into the spindle in rotation and circulate it in the spindle, thereby making the unit complicated and expensive. In the lubricating oil filling method, as described in detail with reference to the conventional spindle unit shown in FIG. 4, the volume of the oil pool can change when the draw bar is moved forward and backward. Thus, a problem is posed in which, due to a long operation, the lubricating oil is pushed out of the oil pool and gradually reduced to such an extent that the clamping means cannot be reliably lubricated.